This invention relates to method and apparatus for instantaneously assessing the stored energy capacity of single, two-volt, lead-acid cells, and of batteries comprised of such cells. More specifically, it relates to specific adaptations of dynamic conductance testing technology, previously developed for determining available cranking power of automotive starting batteries, to the assessment of stored energy capacity of deep-cycle batteries and of their individual cells. Dynamic conductance technology specifically applicable to assessing cranking ability has been disclosed previously in U.S. Pat. Nos. 3,873,911, 3,909,708, 4,816,768, 4,825,170, 4,881,038, and 4,912,416 issued to Keith S. Champlin.
So-called "deep-cycle" lead-acid batteries are used in many applications requiring energy to be delivered continuously over relatively long periods of time. Such batteries, comprises of banks of series-connected two-volt cells, are used at electric generating plants, substations, telephone central offices, railroad signal sites, airport control towers, and countless other critical installations to provide secondary emergency power for use in the event of failure of a primary energy source. Applications requiring relatively long-term reliance on such secondary batteries include emergency lighting for hospitals and industrial plants, and uninterruptible energy supplies for critical communications equipment and computers. Individual cells of secondary batteries are often separate entities with accessible terminals. Such cells may be physically large and will sometimes weight many hundreds of pounds.
The primary mission of a secondary battery system is to supply a specific amount of energy, delivered over a period of hours. In many such applications, it is very desirable that each component cell or battery of the system be periodically tested in order to ensure that it will indeed be capable of delivering its assigned energy if, and when it is called upon to do so. Any cell or battery that is tested and found to have an inadequate energy capacity can then be replaced to ensure that the overall system is capable of fulfilling its role as an emergency energy source.
At the present time, the only available means for accurately assessing the energy capacity of a battery, or of an individual battery cell, is the timed-discharge test. This well-established testing procedure is fully described in Section 6 of ANSI/IEEE Standard 450--1987. Under this procedure, the battery is discharged with a fixed current; usually taken equal to the battery's ampere hour rating divided by its rated time (typically eight or ten hours). During the discharge, the terminal voltage of the battery and of each individual cell is monitored; and the time required to reach a particular "endpoint" voltage (usually 1.75 volts per cell) is recorded. A battery or individual cell's "Percent Capacity" may then be calculated from the formula: ##EQU1## Any cell or battery whose "Percent Capacity" is determined by this procedure to be 80% or less will generally be removed from service and replaced with a new cell or battery.
Although the conventional timed-discharge test described above has been widely used to assess energy storage capacity, it possesses several serious disadvantages. These include:
1. The test takes considerable time to perform (usually 8 or 10 hours).
2. Currents drawn may be relatively large and can thus require apparatus that is heavy and cumbersome.
3. After being tested, the battery must be recharged before it can be returned to service. This requires additional time.
4. Only a fixed number of charge-discharge cycles can be provided by a given battery. As a result, each timed-discharge test performed upon a battery removes potential service capability.
The possibility for developing an alternative to timed-discharge testing of cells and batteries has been suggested by the work of DeBardelaben (s. DeBardelaben, Intelec 86, Toronto, Canada, pp. 365-368). Using laboratory test equipment, DeBardelaben measured the complex impedance of lead-antimony telephone cells rated at 7000 ampere-hours. His analysis, which employed the mathematical technique of linear regression, disclosed a strong correlation between cell capacity and either the magnitude of cell impedance or its resistive real part. Further laboratory studies by Vaccaro and Casson (F. J. Vaccaro and P. Casson, "Internal Resistance: harbinger of Capacity Loss in Starved Electrolyte Sealed Lead Acid Batteries", Intelec 87, Stockholm, Sweden, pp. 128-131) showed that increased impedance and resistance were also good indicator of "dryout" of sealed-lead acid stationary batteries.
Testing of automotive batteries used in engine starting applications presents an entirely different problem. Unlike the deep-cycle battery's mission of supplying energy over an extended period, the primary mission of an automotive starting battery is to supply a large burst of power for a short duration of time. Accordingly, automotive batteries are conventionally tested by means of a short-duration (e.g., 15 second) load test. However, the load test, like the timed-discharge test, also requires heavy, cumbersome, equipment and suffers from other serious disadvantages. Accordingly, a practical alternative to the common load test of automotive starting batteries is taught in U.S. Pat. No. 3,873,911, U.S. Pat. No. 3,909,708, and U.S. Pat. No. 4,816,768. These three patents disclose self-contained electronic apparatus employing small-signal ac measurements of the battery's dynamic conductance (i.e., the real part of its complex admittance) to conveniently and accurately assess an automotive battery's ability to supply cranking power. The patents teach that a battery's dynamic conductance is directly proportional to its dynamic power; the maximum power that the battery can deliver to a load. Measurements of dynamic conductance correlate strongly with a battery's power rating expressed in Cold Cranking Amperes (CCA) and therefore provide a direct measure of the battery's high-current cranking capability. Virtually millions of measurements performed on automotive starting batteries over the course of fifteen years have fully corroborated these teaching and have proven the validity of the dynamic conductance method for testing engine-starting batteries.
Unfortunately, the dynamic conductance method of assessing cranking power cannot be directly applied to the assessment of energy capacity, as would be desired for batteried in deep-cycle applications. Because of the many disadvantages to the timed-discharge test however, it would be obviously desirable to provide a simple, instantaneous, test -- such as a dynamic conductance test -- that could be used to assess stored energy capacity without requiring that the battery be discharged in the process. However, no simple relationship has heretofore been recognized between a cell's dynamic conductance and its stored energy capacity or ampere-hour rating. Thus, it is not obvious a priori that small-signal measurements of a cell's dynamic conductance could be easily related to its stored energy capacity in any meaningful way.
In addition, the dynamic conductance testing apparatus disclosed in the three U.S. Pats. cited above all derive the power required by their electronic circuits from the 6-volt or 12-volt automotive battery undergoing test. This desirable feature permits these dynamic conductance testers to be conveniently used in the field, entirely independent of the ac mains. However, the terminal voltage of a single, fully-charged, lead-acid cell is only about 2.1 volts -- a voltage that is insufficient to power the electronic circuitry disclosed in the cited patents. Furthermore, because of the extremely large conductance of many secondary battery cells, several amperes of ac current would be required to pass through the cell in order to develop sufficient ac voltage to accurately measure dynamic conductance, If this current were to be derived from on-board batteries, these batteries would, of necessity, be large or short-lived. Moreover, any connections to external power sources could adversely effect the isolation required between the "current-feedback loop" and the "voltage-sensing loop" of the measuring circuit's "four-point probe" architecture. Any coupling resulting from such additional power connections could seriously degrade the measuring circuit's ability to suppress errors associated with spurious lead-wire resistance.
Accordingly, even if a simple relationship between dynamic conductance and stored energy capacity could be established, it is not at all obvious how the previously disclosed dynamic conductance testing apparatus could be adapted to test single cells without introducing excessive measurement errors and without necessitating the use of either a large auxiliary battery supply or a separate connection to the ac mains.